The performance of photovoltaic solar arrays and the underlying solar cell devices is typically characterized by the solar electric conversion efficiency. The solar electric conversion efficiency is governed, in turn, by the amount of current induced by incident light and the operating voltage of the device. In traditional single-junction semiconductor solar cells, the conversion efficiency is typically limited to less than 25% of the incident solar power. Photons with energies below the band-gap of the semiconductor pass right through the device and do not contribute to the photo-generated current. High energy photons can be absorbed, but the resulting electrons are collected and extracted at a lower voltage limited by the semiconductor band-gap. Any difference in energy between the photon and the semiconductor energy gap is lost as heat.
Multijunction cells increase the operating voltage, and hence the efficiency, of traditional solar cells by stacking multiple p-n junctions of different semiconductor materials into one two-terminal device. State-of-the-art solar cells have achieved impressive levels of performance in recent years by serially connecting three p-n junctions in one monolithic structure. The best InGaP/GaInAs/Ge three-junction cells have demonstrated one-sun solar conversion efficiencies over 30%, and now exceed 40% under concentrated light. Even higher conversion efficiencies are being pursued by stacking as many as six junctions in one device structure. While initially developed for space power applications, advanced multijunction III-V solar cells are also beginning to make an impact on the terrestrial market. Concentrator photovoltaic systems, which replace expensive semiconductor materials with cheaper plastic lenses and/or metal mirrors, can reduce overall photovoltaic module costs while improving performance, particularly when employing high efficiency III-V cells.
As an alternative to the bulk, multiple-junction approach for achieving high conversion efficiencies, researchers at Imperial College in London have advocated inserting quantum wells into the depletion region of a single-junction solar cell. With this approach, the absorption edge of the solar cell can be extended to lower photon energies, increasing the photon generated current while, ideally, maintaining a high operating voltage. For more on quantum-well solar cells, see U.S. patent application Ser. No. 10/841,843 to Barnham et al., incorporated herein by reference in its entirety.
Suzuki and others have expanded the nanostructured solar cell concept to include quantum dots in addition to quantum wells inside the junction. Ultimately, Marti and co-workers have pointed out that by harnessing a two-step photon absorption process, a single-junction quantum dot solar cell can, in theory, have conversion efficiencies exceeding 60%, which equals or exceeds the theoretical performance of most practical multijunction devices.
Previous reports suggest that at low bias, the dark current of AlGaAs-based PIN photodiodes varies with the location of GaAs wells. At higher bias, however, the insertion of GaAs wells leads to a marked degradation in the dark current. In addition, embedding wide InGaAs wells in InP diodes results in diodes with higher dark currents than structures employing thin wells. Thus, at least one belief in the research community is that “simply using an extended region of narrow band-gap material rather than quantum wells would lead to severe voltage degradation,” as expressed in A. Alemu et al., “Dependence of Device Performance on Carrier Escape Sequence in Multi-Quantum-Well p-i-n Solar Cells,” J. App. Phys., vol. 99, no. 084506, May 2006, incorporated herein by reference in its entirety. Regardless of whether wells or dots are employed in a nanostructured solar cell, it is now believed two critical physical phenomena should be realized in order to achieve any improvements in efficiency: first, the carriers generated by photon absorption in the lower energy gap material should be able to escape before recombining; and second, the diode dark current should not be significantly degraded by the insertion of narrow band-gap material.
Ragay et al. previously proposed an alternative solar cell device with wide band-gap material and narrow band-gap material in F. W. Ragay, “GaAs—AlGaAs Heterojunction Solar Cells with Increased Open-Circuit Voltage,” First World Conference on Photovoltaic Energy Conversion, pp. 1934-1937, December 1994, incorporated herein by reference in its entirety. In the Ragay device, however, the transition from the wide band-gap material to the narrow band-gap material occurs outside of the depletion region. Thus, a built-in field cannot be used to assist the thermionic emission of photogenerated carriers over the barrier in the Ragay structure.
Therefore, a need exists for a solar cell that overcomes or minimizes the above-referenced problems.